1. Field of Invention
This invention relates to a stringer of decorative lights.
2. Background--Description and Evolution of Prior Art
Prior art has seen the development of a wide variety of approaches to forming a stringer of decorative lights, as for showrooms, Christmas trees, and the like.
Originally, stringers of decorative lights utilized line voltage designed incandescent lamps with a screw shell base. The stringers of decorative lights utilized a plurality of matching screw shell sockets connected in a parallel wired circuit. If a lamp were to be removed, or failed during use, the remaining lamps would continue to operate properly. While this wiring circuitry provided a stringer of lights that were relatively easy to maintain, the incandescent lamps that were utilized consumed large amounts of power, and, by design, produced heat. One of the problems associated with the use of these stringers of lights, if applied to a cut live tree, was the rapid drying out of branches and needles. Additionally, if a lamp were to be removed from these stringers during operation, the user could accidentally come in contact with the line voltage due to the exposed electrical contacts of the lamp socket.
Subsequent improvements incorporated the use of miniature incandescent lamps connected in a series wired circuit. (U.S. Pat. No. 3,609,643 to Conan (1971).) While these miniature incandescent lamps reduce the amount of over all heat produced, the series wired circuitry substantially increased the amount of maintenance. This is because in a series wired stringer of lights, when one lamp is removed, or fails to operate, the entire stringer will not operate. This makes it very difficult to locate a faulty lamp or lamps in this type of light stringer.
Further improvements utilize a series-parallel wired circuit to operate the miniature incandescent lamps. (U.S. Pat. Nos. 4,631,650 (1986) and 4,799,177 (1988) both to Ahroni.) While this type of circuitry reduces the number of lamps affected by the removal of a lamp, or failure of a lamp, it does not eliminate this problem entirely.
The difficulty in locating a defective lamp is not the only problem common to both the series and the series-parallel wired stringers of decorative lights. The variety of circuitry types and stringer lengths that are available has led to the use of miniature incandescent lamps that have many different operating voltages. Many of these miniature incandescent lamps utilize the same style and size of lamp base. This can leave the user with incompatible, or non-standardized, lamps and light stringers. Due to the similarity of style, these can be misused and create a potentially dangerous safety hazard.
Additionally, should a series or series-parallel wired stringer of lights experience a short circuit creating an over-voltage, the incandescent lamps remaining within the short circuit absorb this over-voltage condition. This causes the filament of the incandescent lamps to overheat or in some instances, to superheat. This creates the potential for the glass envelope of the miniature incandescent lamp to explode. The temperature and fire ratings of the materials used for the lamp bases, lamp sockets, electrical wire insulation, etc. can be exceeded by such a short circuit condition. The amount of damage to these items, as well as any surrounding materials, is dependent upon the location of the short circuit in relation to its power source, as well as, the length of time these items remain in the short circuit condition. Even the double fuse protection that is found today in the male cord caps of the newer versions of series and series-parallel wired incandescent light stringers, can not fully protect these types of light stringers from all potential short circuits.
A March 1997 report from the National Fire Protection Association (NFPA) states that "the leading cause of Christmas tree fires and property damage was short circuits and ground faults, with roughly 1 in 4 fires". In the U.S. alone, an annual average of 100 fires, 9 deaths, 20 injuries and $5.1 million in property damage can be directly attributed to stringers of lights that experience short circuits or ground faults, states the same report.
More recent developments have seen the introduction of light emitting diodes (LED's) to form a stringer of lights. (U.S. Pat. No. 4,984,999 to Leake (1991) and U.S. Pat. No. 5,495,147 to Lanzisera (1994).) A low voltage transformer is utilized to power a parallel wired circuitry to operate the LED's. While this type of light stringer provides a safer, low voltage, energy efficient stringer of lights, in practical applications such as fully decorating a tree, its limited in its overall length and to the number of LED's by the voltage-amperage size, or output, of the low voltage transformer. Additionally, these low voltage transformers, when loaded, produce a substantial amount of heat.
The series and series-parallel wired light stringers are limited in the way additional light stringers are connected to the initial set. This limits the number of lighting design options available to the user when utilizing these types of light stringers.
From the foregoing it can be seen that prior art fails to provide a stringer of lights that is energy efficient, low heat producing, and easy to maintain. Such a stringer of lights is needed. A stringer with improved safety and short circuit protection is needed. It should be capable of using different sources of illumination, while providing a new flexibility in decorative lighting design.